Plastic and metal containers have been replacing glass in bottling beverages where easy handling, low weight and non-breakability are needed. Plastic packaging, especially polyethylene terephthalate (PET) bottles, are widely used for the packaging of carbonated products such as beer, soft drinks, still waters and some dairy products. For each of these products there is some optimum amount of carbonation or carbon dioxide (sometimes referred to in this document as “CO2”) pressure within the package to maintain its optimum quality. In conventional plastic packaging, it is difficult to maintain the CO2 pressure at this optimum level for an extended period of time.
Plastic packaging is permeable to CO2 and over time the pressure within the bottle diminishes. Ultimately, after a defined amount of carbonation is lost, the product is no longer suitable for use which is usually determined by a noticeable and unacceptable change in flavor or taste. The point at which this occurs generally defines the shelf-life of the package. The CO2 loss rate is highly dependent on the weight and dimensions of the package and on the temperature at which it is stored. Lighter, thinner bottles lose carbonation more quickly, cannot withstand high internal pressures, and have shorter shelf-lives. As plastic bottles become smaller, the relative rate of carbonation loss becomes more rapid. Permeation is faster at higher temperatures, reducing shelf-life, and making it difficult to store carbonated beverages in plastic containers in hot climates and still maintain a reasonable shelf-life. Longer shelf-life, lighter, less expensive plastic bottles, and the ability to store bottles longer in the absence of cooling have numerous economic advantages.
A variety of approaches have been applied to the problems described above. A simple method for extending the shelf-life of a carbonated beverage is to add additional carbon dioxide at the point of filling. This is currently used for carbonated soft drinks and for beer, but its effectiveness is hindered due to the effect of the over-carbonation on product quality and the negative effects that this can cause on the bottle's physical performance. Small differences in internal pressure within the package cause significant differences in the effervescent qualities of the beverage. Dissolved CO2 also effects taste. These precise requirements vary from product to product.
Over-carbonation is also hindered by the pressure limitations of the package. Making the bottle more pressure resistant is possible but requires use of additional material in the bottle construction or more exotic higher performing plastics.
Carbonation can be maintained by reducing the CO2 permeation rate. This typically involves application of a secondary barrier coating to a PET bottle, use of a more expensive, less permeable polymer than PET, fabrication of multilayer bottle constructions, or combinations of these methods. These manufacturing approaches are invariably significantly more expensive than what is incurred in typical polyester bottle production and often these create new problems especially with recycling.
Carbon dioxide generating materials have been used in the art to extend the shelf life of carbonated beverages. Molecular sieves treated with carbon dioxide have been used to carbonate beverages by the reaction of the bound carbon dioxide with water.
U.S. Pat. No. 6,852,783 issued to Hekal and U.S. Patent Application 2004/0242746 A1 to Freedman et al. describes a CO2 releasing composition that can be incorporated or inserted into the packaging for carbonated beverages. The compositions in these references describe over twenty-five percent by weight of inorganic carbonate as the source of the carbon dioxide blended into the thermoplastic. A 32 g PET bottle with a 25% loading of sodium bicarbonate has the potential to release 4.5 grams of carbon dioxide. This is approximately ten times higher than needed for application in a PET beer bottle and would likely cause an unsafe pressurization of the package. These structures also release their carbon dioxide too quickly to regulate pressure over a prolonged period especially if they were prepared in polyethylene terephthalate as opposed to polyethylene which has a far lower permeation rate for moisture. We have found such high loading levels to be unsuitable for our application since they have the potential to release far too much carbon dioxide into the package.